ABSTRACT How pulmonary neuroendocrine cells (PNECs) become innervated and to what extent nerves regulate PNEC function in development and diseases are open and basic questions for the field. The objective of this proposal is to address these fundamental issues and to assess the key role of neurotrophin 4 (NT4) in these events. Using a NT4-/- mouse line and a neonatal mouse model of allergen exposure, we generated preliminary data leading to 3 hypotheses that will be studied in this proposal: 1) NT4 is required for PNEC innervation during development and for the increases in PNEC innervation following early life insults; 2) early life allergen exposure alters the function of sensory afferents and efferent nerves that innervate PNECs thereby causing deregulated ?-aminobutyric acid (GABA) secretion and long-term goblet cell metaplasia; 3) pulmonary mast cells are a candidate source of elevated NT4 levels following early life allergen exposure. To summarize, we found that NT4 was expressed by PNECs during postnatal development and acted as a trophic factor for the innervating nerves to establish connection. Allergen exposure to developing, postnatal lungs aberrantly elevated the levels of NT4. Under this pathological condition, we discovered that PNEC innervation was increased associated with prolonged goblet cell metaplasia. Notably, PNECs were the only cell source of GABA in lungs, a signal essential for allergen-induced goblet cell metaplasia in mouse models and associated with mucous overproduction in human asthmatics and smokers. In addition, NT4 was required for PNEC hyperinnervation and deregulated GABA secretion, consistent with the established paradigm in other neuroendocrine systems that nerves regulate endocrine secretion. These preliminary findings point to an essential role for NT4 in PNEC innervation during development and neuroplasticity following early life injury, which will be extensively characterized by comparing the pattern and degree of PNEC innervation between wild type and NT4-/- mice with and without allergen exposure using markers for different types of nerves (Aim 1). To connect NT4-induced PNEC hyperinnervation to prolonged goblet cell metaplasia following early life allergen exposure, proposed experiments in Aim 2 will assess functional changes in sensory afferents and efferent signals that induce GABA secretion from PNECs and their relationships to NT4. Lastly, given the central role of NT4 in aberrant PNEC innervation under pathological conditions, we examined NT4 expression in injured lungs. We found an enlarged, activated mast cell population expresses NT4 during early life allergen exposure. Whether pulmonary mast cells contribute to PNEC hyperinnervation by producing NT4 will be evaluated (Aim 3). To further enhance disease relevance, we will validate key findings from the mouse work in infant primate models of injury and human lung samples. Together, this proposal investigates complex interactions between nerves, PNECs, and inflammation during postnatal development and injury. Our findings indicate that the pathogenesis of chronic airway diseases, such as asthma, may involve disrupted developmental processes following early episodes of insults. We expect that completion of the proposed studies will provide fundamental knowledge about how the pulmonary neuroendocrine system forms and functions. Identification of the mechanisms along the nerve-PNEC axis underlying mucous overproduction may lay the foundation for the discovery of new treatment strategies.